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WORLD METEOROLOGICAL ORGANIZATION
COMMISSION FOR BASIC SYSTEMS
OPAG ON INFORMATION SYSTEMS & SERVICES
Guide for Virtual Private Networks (VPN) via the Internet between GTS centres
TABLE OF CONTENT
1
INTRODUCTION .................................................................................................................................................... 3
2
WHAT IS A VPN ? .................................................................................................................................................. 4
2.1
DEFINITION ........................................................................................................................................................ 4
2.2
TYPES OF VPNS ................................................................................................................................................. 5
2.2.1 The Link Layer solutions ............................................................................................................................... 5
2.2.1.1
2.2.1.2
2.2.1.3
2.2.2
Transport and application Layer .................................................................................................................. 7
2.2.2.1
2.2.2.2
2.2.2.3
2.2.3
3
SSL and TSL .........................................................................................................................................................7
SSH........................................................................................................................................................................8
SOCKS ..................................................................................................................................................................8
So, why IPSEC ? ........................................................................................................................................... 8
WHAT IS IPSEC ? ................................................................................................................................................ 10
3.1
3.2
3.3
3.4
3.5
3.6
3.7
4
ATM and Frame Relay ..........................................................................................................................................5
MPLS ....................................................................................................................................................................6
PPTP and L2TP .....................................................................................................................................................6
IPSEC ARCHITECTURE ...................................................................................................................................... 10
IPSEC SERVICES AND MODES ........................................................................................................................... 11
SECURITY ASSOCIATIONS ................................................................................................................................. 13
AUTHENTICATION HEADER - AH...................................................................................................................... 13
ENCAPSULATING SECURITY PAYLOAD - ESP ................................................................................................... 14
INTERNET KEY EXCHANGE – IKE .................................................................................................................... 14
CONCLUSION .................................................................................................................................................... 15
IPSEC AND THE GTS .......................................................................................................................................... 16
4.1
CRYPTOGRAPHY AND THE LAW ........................................................................................................................ 16
4.2
A QUICK VIEW ON THE GTS ............................................................................................................................. 17
4.2.1 Physical layer ............................................................................................................................................. 17
4.2.2 Upper layers ............................................................................................................................................... 17
4.3
THE INTERNET ................................................................................................................................................. 17
4.4
THE SUGGESTED APPROACH ............................................................................................................................. 17
4.5
SOLUTION SUPPLIERS ....................................................................................................................................... 18
4.6
IMPLEMENTATION SCENARIO............................................................................................................................ 19
5
OPERATORS SOLUTIONS................................................................................................................................. 20
5.1
STAR, PARTIALLY MESHED, FULLY MESHED NETWORKS ................................................................................... 20
5.2
TECHNICAL SOLUTIONS .................................................................................................................................... 20
5.2.1 MPLS .......................................................................................................................................................... 21
5.2.2 IPSec VPN .................................................................................................................................................. 21
5.3
CONCLUSION .................................................................................................................................................... 21
6
APPENDICES ........................................................................................................................................................ 22
6.1
A CISCO BASED CONFIGURATION ..................................................................................................................... 22
6.1.1 The test platform ......................................................................................................................................... 22
6.1.2 ADSL connection with NAT ........................................................................................................................ 22
6.1.3 VPN between the routers ............................................................................................................................ 23
6.1.3.1
6.1.3.2
6.1.3.3
IKE ......................................................................................................................................................................25
AH et ESP ...........................................................................................................................................................26
Security Association ............................................................................................................................................26
6.1.4 Final configuration ..................................................................................................................................... 26
6.2
GLOSSARY ....................................................................................................................................................... 29
CBS/ISS/VPN via the Internet, p. 2
BLANK
CBS/ISS/VPN via the Internet, p. 3
1 Introduction
After describing various concepts related to VPN and IPSEC, this document presents a potential methodology to introduce the technical solution on the GTS and presents why these tools can enhance communication capabilities among
WMO Members for operational traffic exchanges.
Chapter 2 introduces the concept of VPN and presents various technical alternatives to IPSEC. As IPSEC is the most
appropriate protocol for secure network-to-network communication the document will then focus on this solution.
Chapter 3 briefly describes what is IPSEC. IPSEC is not a simple solution to understand. It covers a lot of technical aspects, allows various flavors of algorithms, and is better describe as a framework instead of a protocol.
Chapter 4 focuses on application of IPSEC on the GTS. A selection of protocols is proposed. It shows what benefits
WMO Members could expect from the use of IPSEC.
Chapter 5 shows the technical evolution of IP services from the operators. Evolving from Frame Relay to IP solutions
operators offer VPN solutions either MPLS based or IPSEC based.
Chapter 6 gives :
- a complete configuration example on Cisco routers with the protocols selected in chapter 3.
- a glossary of terms related to VPN
- some references on IPSEC
CBS/ISS/VPN via the Internet, p. 4
2 What is a VPN ?
2.1 Definition
The definition below come from “What is a VPN” – P.Ferguson and G.Huston
Perhaps the simplest method of attempting to arrive at a simple definition for VPN’s is to look at each word in the acronym individually, and then subsequently tie each of them together in a simple, common sense, and meaningful fashion.
Let’s start by examining the word “network” This is perhaps the least difficult term for us to define and understand,
since the commonly accepted definition is fairly uncontroversial and generally accepted throughout the industry. A network consists of any number of devices which can communicate through some arbitrary method. Devices of this nature
include computers, printers, routers, and so forth, and may reside in geographically diverse locations. The methods in
which they may communicate are numerous, since there are countless electronic signaling specifications, and data-link,
transport, and application layer protocols. For the purposes of simplicity, let’s just agree that a “network” is a collection
of devices that can communicate in some fashion, and can successfully transmit and receive data amongst themselves.
The term “private” is fairly straightforward, and is intricately related to the concept of “virtualization” insofar as VPN’s
are concerned, as we’ll discuss in a moment. In the simplest of definitions, “private” means that communications between two (or more) devices is, in some fashion, secret – that the devices which are not participating in the “private” nature of communications are not privy to the communicated content, and that they are indeed completely unaware of the
private relationship altogether. Accordingly, data privacy and security (data integrity) are also important aspects of a
VPN which need to taken into consideration when considering any particular VPN implementation.
Another means of expressing this definition of "private" is through its antonym, "public." A “public” facility is one
which is openly accessible, and is managed within the terms and constraints of a common public resource, often via a
public administrative entity. By contrast, a “private” facility is one where access is restricted to a defined set of entities,
and third parties cannot gain access. Typically, the private resource is managed by the entities who have exclusive right
of access. Examples of this type of private network can be found in any organizational network which is not connected
to the Internet, or to any other external organizational network, for that matter. With this definition the current GTS is a
private network
These networks are private due to the fact that there is no external connectivity, and thus no external network communications. Another important aspect of “privacy” in a VPN is through its technical definition, as describing the privacy of
addressing and routing system, meaning that the addressing used within a VPN community of interest is separate and
discrete from that of the underlying shared network, and from that of other VPN communities. The same holds true for
the routing system used within the VPN and that of the underlying shared network. The routing and addressing scheme
within a VPN should, for all intents and purposes, be self-contained, but this degenerates into a philosophical discussion
on the context of the term “VPN.”
“Virtual” is a concept that is slightly more complicated. The New Hacker’s Dictionary [2] defines virtual as –
virtual /adj./ [via the technical term “virtual memory”, prob. from the term “virtual image” in optics] 1. Common alternative to {logical}; often used to refer to the artificial objects (like addressable virtual memory larger than physical
memory) simulated by a computer system as a convenient way to manage access to shared resources. 2. Simulated; performing the functions of something that isn’t really there. An imaginative child’s doll may be a virtual playmate. Oppose
{real}.
Insofar as VPN’s are concerned, the definition in 2. above is perhaps the most appropriate comparison for virtual networks. The “virtualization” aspect is one that is similar to what we briefly described above as “private,” however, the
scenario is slightly modified – the private communication is now conducted across a network infrastructure that is
shared by more than a single organization. Thus, the private resource is actually constructed by using the foundation of a
logical partitioning of some underlying common shared resource, rather than by using a foundation of discrete and dedicated physical circuits and communications services. Accordingly, the “private” network has no corresponding “private”
physical communications system. Instead, the “private” network is a virtual creation which has no physical counterpart.
The virtual communications between two (or more) devices is due to the fact that the devices which are not participating
in the virtual communications are not privy to the content of the data, and that they are also altogether unaware of the
private relationship between the virtual peers. The shared network infrastructure could, for example, be the global Inter-
CBS/ISS/VPN via the Internet, p. 5
net and the number of organizations or other users not participating in the virtual network may literally number into the
thousands, hundreds of thousands, or millions.
A VPN can also said to be a discrete network –
discrete \dis*crete"\, a. [L. discretus, p. p. of discernere. See Discreet.] 1. Separate; distinct; disjunct.
The discrete nature of VPN’s allow both privacy and virtualization. While VPN’s are not completely separate, per se,
the distinction is that they operate in a discrete fashion across a shared infrastructure, providing exclusive communications environments which do not share any points of interconnection.
The combination of these terms produces VPN – a private network , where the privacy is introduced by some method
of virtualization. A VPN could be built between two end-systems or between two organizations, between several endsystems within a single organization or between multiple organizations across the global Internet, between individual
applications, or any combination of the above.
The common and somewhat formal characterization of the VPN, and perhaps the most straightforward and strict definition, is:
A VPN is a communications environment in which access is controlled to permit peer connections
only within a defined community of interest, and is constructed though some form of partitioning of
a common underlying communications medium, where this underlying communications medium
provides services to the network on a non-exclusive basis.
This definition introduces a concept, the VPN, not related to any technical implementation.
There are quite a lot of technical implementations of VPNs.
2.2 Types of VPNs
A simplified version of the TCP/IP layer model is shown on left.
The technical implementation of the VPNs are related to this model :
- On the link layer one can find :
o ATM and Frame Relay connection
o MPLS (Multi Protocol Label Switching)
o Link-Layer Encryption (L2TP or PPTP)
- On the network layer :
o IPSEC
- On the transport and application layer
o SSL (Secure Socket Layer) is a protocol proposed by Netscape mainly
for http traffic encryption
o TSL (Transport Secure Layer) is a proposed standard by IETF (Internet
Engineering Task Force) based on SSL
o SOCKS
o SSH
2.2.1 The Link Layer solutions
2.2.1.1 ATM and Frame Relay
Following the definition of VPNs gave on 1. ATM and Frame Relay solution must be considered as VPNs. By construction a Frame Relay (and ATM) network, like the RMDCN is a VPNs. The telco Equant has a network, which is securely
CBS/ISS/VPN via the Internet, p. 6
divided among all the customers. Therefore, on a global telecommunication system, coexist multiple isolated “sub“networks. In this case, the VPNs rely on the operator.
2.2.1.2 MPLS
Nowadays, as IP is becoming the base protocol, most of the telco offers are moving to MPLS. Multi Protocol Layer
Switching is a protocol originated by Cisco (the Tag Switching initiative), but now widely adopted.
The chart below briefly summarized the main concepts of MPLS.
In the traditional IP world, every router must route every packet on the network. Routing is rather complex and by the
way slow. MPLS introduce (or use) the concepts of tags. Packets are “tagged” at the “entrance” of the WAN. Inside the
WAN packets are switched (not routed) based on the tag. Tags are removed at the network exit.
This solution is now widely offered by operators.
The last link layers VPN solution described in this document are L2TP and PPTP. L2TP (Layer 2 Tunnelling Protocol )
and PPTP (Point to Point Tunnelling Protocol) are two solutions mainly dedicated to remote access. In the “normal” situation a remote user who wants to connect to the intranet use a PPP connection to a Remote Access Server. In this case,
username and password (and also data) are transferred in plain text and therefore might be “sniffed” by potential intruders. L2TP and PPTP permit to encrypt traffic between peers leading to better security
2.2.1.3 PPTP and L2TP
2.2.1.3.1 Point-to-Point Tunneling Protocol (PPTP)
PPTP is a Layer 2 protocol that encapsulates PPP frames in IP datagrams for transmission over an IP internetwork, such
as the Internet. PPTP can be used for remote access and router-to-router VPN connections. PPTP is documented in RFC
2637.
The Point-to-Point Tunneling Protocol (PPTP) uses a TCP connection for tunnel maintenance and a modified version of
Generic Routing Encapsulation (GRE) to encapsulate PPP frames for tunneled data. The payloads of the encapsulated
PPP frames can be encrypted and/or compressed. Figure 6 shows the structure of a PPTP packet containing user data.
CBS/ISS/VPN via the Internet, p. 7
2.2.1.3.2 Layer Two Tunneling Protocol (L2TP)
L2TP is a combination of PPTP and Layer 2 Forwarding (L2F), a technology proposed by Cisco Systems, Inc. L2TP
represents the best features of PPTP and L2F. L2TP encapsulates PPP frames to be sent over IP, X.25, Frame Relay, or
Asynchronous Transfer Mode (ATM) networks. When configured to use IP as its datagram transport, L2TP can be used
as a tunneling protocol over the Internet. L2TP is documented in RFC 2661.
L2TP over IP internetworks uses UDP and a series of L2TP messages for tunnel maintenance. L2TP also uses UDP to
send L2TP-encapsulated PPP frames as the tunneled data. The payloads of encapsulated PPP frames can be encrypted
and/or compressed.
In Windows 2000, IPSec Encapsulating Security Payload (ESP) is used to encrypt the L2TP packet. This is known as
L2TP/IPSec. The result after applying ESP is shown below
2.2.2 Transport and application Layer
These layers covers mainly host based solution.
2.2.2.1 SSL and TSL
Netscape, few years ago, created the protocol SSL (Secure Socket Layer). In the TCP/IP layering model it is on top of
the TCP layer.
Therefore, it could be use for adding security (that is strong authentication and encryption) for all TCP-based application (Telnet FTP…).
Some implementations exist for these protocols but the success story of SSL is HTTPS. HTTPS is used in e-commerce
application to allow secure information exchanges between client and servers. TSL is the IETF proposed standard
equivalent to SSL.
CBS/ISS/VPN via the Internet, p. 8
2.2.2.2 SSH
SSH (Secure Shell) is another application layer authentication and encryption protocol. The SSH FAQ (Frequently
Asked Question) give the following definition of SSH :
Secure Shell is a program to log into another computer over a network, to execute commands in a remote machine, and to move files from one machine to another. It provides strong authentication and
secure communications over unsecure channels. It is intended as a replacement for telnet, rlogin, rsh,
and rcp. For SSH2, there is a replacement for FTP: sftp.
Therefore, the main use of SSH is within organizations. In theory to manage security based devices (firewalls…), or to
gain root access on hosts the network/system administrator should avoid to connect remotely using telnet to the box. If
telnet is used, it is very easy, with a sniffer, to capture and to analyze the packets to gain administrative access on the
firewall/system. With a direct access no clear user/password will be exchange on the LAN. But, network administrators
are often lazy… SSH is the answer in this case !
SSH among other things includes an encrypted replacement tool for telnet.
SSH is becoming very popular for secure remote management.
2.2.2.3 SOCKS
SOCKSv5 is an IETF (Internet Engineering Task Force) approved standard (RFC 1928) generic, proxy protocol for
TCP/IP-based networking applications. The SOCKS protocol provides a flexible framework for developing secure
communications by easily integrating other security technologies.
SOCKS includes two components, the SOCKS server and the SOCKS client. The SOCKS server is implemented at the
application layer, while the SOCKS client is implemented between the application and transport layers. The basic purpose of the protocol is to enable hosts on one side of a SOCKS server to gain access to hosts on the other side of a
SOCKS Server, without requiring direct IP-reachability.
Socks and the OSI layer model
If SSH is mainly use for secure remote connection, SOCKS is primarily used as a way to provide a secure tunnel between to points and to hide network topology. But, they are both mainly related to client-server exchanges.
2.2.3 So, why IPSEC ?
In the “Guide on the use of TCP/IP on the GTS”, WMO presents two solutions to exchange traffic between MSS using
the IP protocol. One is based on FTP and the other on sockets.
This guide does not cover the WAN infrastructure. The current GTS is a mixed of leased lines, peer-to-peer Frame Relay links, global Frame Relay services (like RMDCN in RA VI). For economical reasons, and in regards the overall
good quality of service of the Internet, it might be a good opportunity to study the potential use of the Internet to complement the GTS.
However, if reliable (but no real SLA –Service Level Agreement-) the Internet is by nature an insecure network. Various
documents within WMO shown how NMCs should connect to the Internet (firewalls…).
CBS/ISS/VPN via the Internet, p. 9
In order to allow a smooth introduction of the Internet to complement the GTS the following rules should apply :
- Permit the use of the current protocols (FTP and socket) on the Internet
- Avoid any impact on the MSS
- Guarantee an acceptable level of trust for members
The two first point means that the proposed solution should be transparent to the application and the hosts. Among the
protocols describe above, IPSec is the only one completely application independent.
To offer a minimum level of trust, authentication (who wants to talk to me) and encryption (no one except me can understand the data) are both needed. IPSec offers these two services.
CBS/ISS/VPN via the Internet, p. 10
3 What is IPSEC ?
3.1 IPSec architecture
From RFC 2401 :
IPSec is designed to provide interoperable high quality, cryptographically-based security for IPv4
and IPv6. The set of security services offered includes access control, connectionless integrity, data
origin authentication, protection against replays, confidentiality, and limited traffic flow confidentiality. These services are provided at the IP layer offering protection for the IP and upper layer
protocols.
The
IPSec
spec
ification is rather complex. The overall architecture of the specification can be seen as a suite of interacting protocols. The
RFC2401 gives the organization of the specifications. It can be seen as :
IPSec document overview
Architecture
RFC 2401
ESP Protocol
RFC 2406
AH Protocol
RFC 2402
Encryption
Algorithm
Authentication
Algorithm
DOI
RFC 2407
Policy
Key
Management
• Architecture covering the general concepts, security requirements, definitions and mechanisms defining IPSec technology.
- defines the capabilities hosts and routers should provide
- for example, it is required that the hosts provide confidentiality using ESP. However this document does
not specify the header format.
- describes the interaction between IPSec and rest of TCP/IP
• Encapsulation security payload ESP and Authentication header (AH)
- define the protocol, the payload header format and the services they provide.
- define the packet processing rules
- do not specify the cryptographic transforms that are used to provide these capabilities. This allows the
transforms to be changed if they become cryptographically insecure without any change in the base protocol.
CBS/ISS/VPN via the Internet, p. 11
• Encryption algorithm and Authentication algorithm
- a set of documents that describe how various encryption algorithms are used in ESP or how various authentication algorithms are used in AH and authentication part of ESP.
- defines the algorithm, the key sizes, the derivation of keys, transformation process, any algorithm-specific
information.
- the definitions have to be very specific in order to obtain interoperability.
• Key management describing the key management schemes.
- keys are generated with Internet Key Exhange (IKE) in IPSec protocols
- The payload format of IKE is very generic. It can be used to negotiate keys in any protocol. IKE is also
used for negotiating keys for other protocols outside IPSec.
- The genericity is achieved by separating the parameters IKE negotiates from the protocol itself.
• Domain of Interpretation (DOI) contains values needed for the other documents to relate to each other, i.e. identifiers
for approved encryption and authentication algorithms, operational parameters like key lifetime.
- the parameters negotiated by IKE are defined in DOI
• Policy is an important component
- determines if two entities will be able to communicate with each other, and if so, which transforms to use.
- Policy representation deals with definition, storage and retrieval of policy.
- Policy implementation addresses the application of policy for actual communication involving e.g. the application of negotiated keys in the communication.
All these documents are RFCs, and as often with RFC, not very readable !!
The following paragraphs introduce IPSec and some key point in it.
3.2 IPSec services and modes
The IPSec framework has been build and defined that way, to guarantee the maximum independence between the different part of the system (encryption algorithm, authentication algorithm… are not linked to ESP and AH protocols for example)
The goal of IPSec is to offer security through encryption and it has been decided to split the solution in several parts.
This lead to a very powerful solution, depending on the goal one wants to reach the “good” protocols may be chosen
among a large choice. However, it may lead to some incompatibility…
It must also been already noted that in the first design (the first sets of RFCs was published in 1995), the ESP protocol
was not usable for authentication. In the second set (published late 1998), the ESP protocol also offers authentication solution.
When two systems want to exchange data using IPSec they must first determine the services they want to use from IPSec. The table below summarize the services offer by ESP and AH.
IPSec Services
AH
ESP (Encryption Only)
ESP (Encryption and
authentication)
Access Control



Connectionless Integrity


Data origin
authentication
Rejection of replayed
packets





CBS/ISS/VPN via the Internet, p. 12
Confidentiality


Limited traffic flow
confidentiality


This is the first step… but not the last !
Next one, Tunnel mode or Transport mode. The transport mode is mainly designed for host-to-host communication
where IPSec is embedded in the host operating system. In Tunnel mode, the host are not in charge of IPSec, some boxes
between the hosts are doing the job.
See below the structure of the packets in the two modes.
IPSec operations within transport mode
Application
Data
Transport
TCP
Header
Data
IP
Header
TCP
Header
Network
Data created from upper layer
Transport information is applied
for end-to end communication
Data
Network information is applied
(Normal Network Layer Information)
IP
Header
TCP
Header
Data
IPSec isolates IP Header
from upper layer data
IP
Header
TCP
Header
Data
IPSec encrypts upper layer data
TCP
Header
Data
IP
Header
IP
Header
ESP/
AH
ESP/
AH
TCP
Header
Data
ESP
TRL
ESP
TRL
IPSec applies the security protocol
The packet is reassembled
In this mode, the real IP header of the packet in used along the way. The IP addresses of the hosts are used to route the
packet. It mean that if the packet is going on the Internet, the IP addresses of both hosts must be routable…
CBS/ISS/VPN via the Internet, p. 13
IPSec operations within tunnel mode
Application
Data
Transport
TCP
Header
Data
IP
Header
TCP
Header
Data
Network information is applied
(Normal Network Layer Information)
IP
Header
TCP
Header
Data
IPSec encrypts datagram
Network
Data created from upper layer
Transport information is applied
for end-to end communication
ESP/
AH
IP
Header
TCP
Header
Data
IP
Header
ESP/
AH
IP
Header
TCP
Header
IP
Header
ESP/
AH
IP
Header
TCP
Header
ESP
TRL
Data
Data
IPSec applies the security
protocol
ESP
TRL
ESP
TRL
IPSec build new
IP header
The packet is reassembled
The real IP addresses are embedded in the new IPSec packet. If the packet is routed over the Internet, the new IP addresses must be routable.
3.3
Security associations
The security associations or SA in IPSec terminology form the basis for IPSec operations. The SAs are the contracts between two entities determining the IPSec protocols used for securing the packets, the transforms, the keys and their duration plus many other things.
Before two entities are able to exchange packets using IPSec they must first create SAs. SA are always one-way (simplex). So if hosts A and B are communicating with IPSec, each host will have two SA : SAin and SAout. SAin for host A
and SAout for host B will share the same cryptographic parameters. Likewise SAs are protocol specific. There is an SA
for each protocol : ESP and AH. Each IPSec host therefore has to keep a database (the SADB) to store all SA, the SPI
(Security Parameter Index) is a 32-bit element used to identified the SA in the SADB.
SA are managed (created and deleted) either manually or with a protocol, and in this case the protocol used is IKE (Internet Key Exchange).
It is out of the scope of this paper to describe the exact and complete use of SAs but, it must be clear that SAs are the
basis of the IPSec framework.
In the framework presented above, SA is linked to “Policy” and “DOI” (Domain Of Interpretation). Security Association
is the link between the concepts (IPSec framework) and the reality (how to really use all these things in data communication).
3.4 Authentication Header - AH
AH (Authentication Header) provides data security and authentication of IP packets :
- data integrity ensures that undetected modification to a packet content in transit is not possible
- authentication feature enables end system or network devices to authenticate the user or the application and
filter traffic accordingly.
CBS/ISS/VPN via the Internet, p. 14
-
prevents the address spoofing attacks
protects against the replay attack
The three first functions are guaranteed by calculation of an authentication hash function. A MAC function (Message
Authentication Code) is an authentication process combined with a symmetrical key. To make it short, such function
calculates a digest of a message, afterwards the digest is encrypted with a shared secret between the two hosts.
A digest is the result of a fixed length mathematical calculation. It has been proven that is was impossible to recreate the
digest if the original information was altered.
Such techniques are used, for example, on public FTP server. When publishing a file on a server, it is useful to also put
the digest of the file on the same server. The digest guarantees that the original file has not been modified by anyone.
The most well-known digest calculation method is called MD5.
The required protocols for AH are :
- HMAC-MD5-96
- HMAC-SHA-1-96
The last function in AH (anti-replay attack) is built with a mechanism of sliding window. Each packet receives a sequence number and various techniques guarantee that packet can not be replayed without notice.
3.5 Encapsulating Security Payload - ESP
ESP offers two services :
- encryption : the data is encrypted with a predefined protocol between the two hosts
- authentication : see AH
In the first release of RFC regarding IPSEC, AH was used for authentication and ESP for encryption only. It has been
shown by further study, that encryption without authentication was not secure. As, RFCs allow using ESP without AH it
was decided to add in the new RFC the capability to authenticate within ESP.
There is now some redundancy ! It is possible to use AH to authenticate, and to use ESP also to authenticate and encrypt. Bruce Shneier, a well-known cryptographic specialist suggest to use ESP for both and to forget about AH…
The authentication algorithm usable in ESP are :
- DES in CBC mode – the only mandatory one
- 3DES
- RC5
- IDEA
- 3 IDEA
- CAST
- Blowfish
DES (Data Encryption Standard) is a rather old protocol developed and promoted by the US government. It has been
proven that DES was no more secure. A “brute force” attack could compromise the security of the data encrypted with
DES.
NIST (National Institute of Standards and technology) has recently promoted a new protocol AES (Advances Encryption Standard). With the design of IPSec (a collection of separated pieces) AES is going to be rapidly the de-facto
standard in IPSec.
3.6 Internet Key Exchange – IKE
CBS/ISS/VPN via the Internet, p. 15
In order to talk to each other, peers must use SA. An SA defines the security parameters and authentication keys to use.
IKE is the protocol used to create SA.
However, nothing simple in IPSec theory… In fact IKE covers three different protocols :
- ISAKMP – Internet Security Association and Key Management Protocol
- OAKLEY
- SKEME
ISAKMP defines the language for negotiation. It defines the payload format, the mechanics of implementing a key exchange and the negotiation of a security association. ISAKMP does not define the key exchange algorithm but rather the
message types in order to exchange keys.
OAKLY and SKEME are two key exchange protocols usable under ISAKMP umbrella in the IKE world.
In fact, ISAKMP, OAKLEY and SKEME were already developed protocol and IKE is a glue between all these.
3.7 Conclusion
In this brief introduction to IPSec, we have seen that :
- Two modes exists : Tunnel and Transport
- Two protocols partially redundant are defined : ESP and AH
- Security Association are created and maintained by three different protocols
- Authentication can be achieve through two possible algorithms
- Encryption rely on DES (which is insecure) and 6 others protocols. A new one AES will probable overcome all others
We have neither covered nor introduced other aspects which may also be relevant in IPSec :
- Diffie-Helmann protocol to create keys
- Certificates
- PKI –Public Key Infrastructure
- …
VPN, in some aspect, is quite a vague concept, IPSec, one of the VPN solution is not much clear !
So, despite the rather ugly face of the protocol, the various options, the risk of incompatibility, IPSec is now widely implemented and available on a lot of platform, including, routers, dedicated boxes, firewalls, hosts…
In fact, in most cases, the solutions implemented only cover a part of the possible options. It is therefore, very important
to guarantee interoperability to share the same subset among the peers.
In the following part, we suggest a possible scenario taking into account the need of security, some legal aspects and interoperability issues.
CBS/ISS/VPN via the Internet, p. 16
4 IPSEC and the GTS
4.1 Cryptography and the law
In VPNs and also in IPSec, cryptography is one of the key point to ensure privacy of the communication. The government in all the countries have and still do consider cryptography as some sort of weapon.
It has often be explained by different government in the world, that lawful activities perhaps needs privacy but, the main
use of crypto tools was for unlawful activities. Therefore, every country has defined rules about the use of cryptography
within the country.
The web site http://rechten.kub.nl/koops/cryptolaw/ presents the situation regarding cryptography in a
lot of countries (from the web site) :
In the last years a group of countries worked on the Wassenaar Arrangement. It controls the export of weapons and of dual-use goods, that is, goods that can be used both for a military and
for a civil purpose; cryptography is such a dual-use good.
In 1995, 28 countries decided to establish a follow-up to COCOM, the Wassenaar Arrangement on Export Controls for Conventional Arms and Dual-Use Goods and Technologies. The
negotiations on the Arrangement were finished in July 1996, and the agreement was signed by
31 countries (Argentina, Australia, Austria, Belgium, Canada, the Czech Republic, Denmark,
Finland, France, Germany, Greece, Hungary, Ireland, Italy, Japan, Luxembourg, the Netherlands, New Zealand, Norway, Poland, Portugal, the Republic of Korea, Romania, the Russian
Federation, the Slovak Republic, Spain, Sweden, Switzerland, Turkey, the United Kingdom and
the United States). Later, Bulgaria and Ukraine also became a participating state to the Arrangement.
The initial provisions were largely the same as old COCOM regulations. The General Software
Note (applicable until the December 1998 revision) excepted mass-market and public-domain
crypto software from the controls. Australia, France, New Zealand, Russia, and the US deviated
from the GSN and controlled the export of mass-market and public-domain crypto software.
Export via the Internet did not seem to be covered by the regulations.
There is a personal-use exemption, allowing export of products "accompanying their user for
the user's personal use" (e.g., on a laptop).
The Wassenaar Arrangement was revised in December 1998. Negotiations were held on 2 and
3 December 1998 in Vienna, which resulted in restrictions on the General Software Note and in
some relexations:
- free for export are: all symmetric crypto products of up to 56 bits, all asymmetric crypto
products of up to 512 bits, and all subgroup-based crypto products (including elliptic curve) of
up to 112 bits;
- mass-market symmetric crypto software and hardware of up to 64 bits are free for export
(the 64-bit limit was deleted on 1 December 2000, see below);
- the export of products that use encryption to protect intellectual property (such as DVDs) is
relaxed;
- export of all other crypto still requires a license
DES protocol uses a 56 bits key. It is therefore legal in most countries. But 3DES, which is equivalent to a 168-bit key,
is often out-of-law.
.
Before any use of cryptographic solution, everyone and especially government
agencies such as National Meteorological Centres MUST verify if the solution they
want to use are legal in their country
CBS/ISS/VPN via the Internet, p. 17
4.2 A quick view on the GTS
4.2.1 Physical layer
The current GTS is a mixed of various technical solutions :
- leased line, the most common but for sure not the less expensive solution in case of international circuits
- Frame-Relay lines. The leased line is replaced by a connection to a Frame Relay network but only two
peers share the same Frame Relay network.
- Frame Relay network. A group of countries share the same operated network and agreed on a technical and
commercial global solution. The RMDCN in region VI is such a network
The reproaches for the three solutions are quite the same :
- costs : for (relatively) slow speed lines the price are high
- either the leased line or the PVCs in Frame Relay are one-to-one link and therefore works well for one-toone communication, but neither for many-to-many nor any-to-any dialog are convenient.
But, the current needs on telecommunication become higher and higher in terms of :
- traffic
- global exchange
With the growth of products and datas, NMCs want to exchange much more information. The current architecture based
on concentration through RTHs and costly slow speed line is not enough opened to allow such an evolution.
4.2.2 Upper layers
The “Guide on the use of TCP/IP on the GTS” describes how to use IP as a replacement of the legacy X25 protocol.
The future of the GTS is IP and technical evolution on the network should be searched in the IP direction.
At the application layer, two protocols are usable above the IP layer : FTP and sockets. Every new solution must conform to these standards.
4.3 The Internet
Most NMCs are now connected to the Internet. The Internet connection is in most cases :
- reliable. The ISP (Internet Service Providers) propose connection to the Internet. They are operated just
like others leased lines and are therefore as reliable as the lines. However it must be noted that no end-toend SLA (Service Level Agreement) could be defined on the Internet.
- powerful. The Internet connection is often a high speed connection.
- secured. The NMCs should (and even must) be protected by firewalls.
Therefore, the Internet is becoming a possible media to complement the current GTS private infrastructure.
4.4 The suggested approach
As seen before, the only real application and host independent VPN solution is IPSec.
Therefore, for WMO use we recommend IPSec as the VPN solution.
But in order to guarantee interoperability between NMCs without redefining each time the protocols to use we suggest
the following implementation solution :
- Tunnel mode : as IPSec will be the most probably configured on routers, firewall or dedicated boxes, and
taking into account that neither encryption nor authentication are mandatory on LAN, Tunnel mode is the
most appropriate solution
- AH ( Authentication Header) should not be used
CBS/ISS/VPN via the Internet, p. 18
-
ESP (Encapsulting Security Paylod) is used for authentication
o Authentication should be done with HMAC-SHA-96
Pre-Shared secrets : Certificated is probably a more elegant solution, but, in practical, more difficult to implement in WMO situation.
For meteorological data exchanges, there is no real reason to use encryption.
These recommendations mostly come
(http://www.counterpane.com/ipsec.html).
from “A
cryptographic
evaluation
of IPSec”
form Couterpane
4.5 Solution suppliers
With the subset of protocols defined above, each NMCs can choose any supplier for VPNs equipment.
Of course, Cisco is a solution.
Cisco offers on VPN is very widespread :
- router with software encryption
- router with hardware incryption
- firewalls
- dedicated boxes
But others, like Nortel, CheckPoint… have also technical solution.
All IPSec implementation should be compatible. But, as seen above, IPSec offers a large choice. The solution proposed
in 4.4 is included in the mandatory subset of protocols described in the RFCs. So any RFC compliant solution should interoperate with others. In very infrequent cases it might not be true. So before choosing a VPN box, NMCs should check
with potential VPN neighbor if their respective solutions are compatible. This information can be found on the web.
4.6 Network architecture
The placement of the VPN gateway in a security solution is of paramount importance. Improper placement of the VPN
gateway will impact effectiveness of the solution. In this context, all the VPN gateways are functioning as tunneling
mode only. The following few scenarios of VPN gateway placement highlight this point. Emphasis is placed on the Intranet infrastructure and the demilitarization zone (DMZ). The DMZ is typically meant for limited authorized access.
Servers that are put in the DMZ include the WEB servers and external Mail servers.
Quite a lot of different solutions are available.
Two solutions are here highlighted :
- the simplest one: firewall and VPN gateway on the same box
- VPN gateway adjacent to the firewall
It must be noted that this architecture must be coherent with the security policy of the site.
In this case a VPN gateway function is implemented on a firewall and the firewall connects to the Internet. The flow of
traffic is from the Internet to the firewall/VPN gateway directly. This will be a more cost effective solution since there is
CBS/ISS/VPN via the Internet, p. 19
no need for a separate VPN gateway. Since both the functions (firewall/VPN) are residing in the same box resulting in
ease of management. Note that this setup is typically used for small VPN solution.
The disadvantage of this solution is that the resource utilization of the firewall/VPN gateway will be very high and adequate measures have to be taken to address this issue. In other words, this setup introduces additional software components in the firewall and hence result in performance degradation. Another disadvantage of this solution is that it requires more services/ports to be opened, as a result, the security implementation may suffer from additional security
holes. Finally, the firewall poses a single point of failure for the VPN solution.
In this case the VPN gateway is connected an interface of a firewall. The flow of traffic into the intranet will be through
the firewall. However, the Internet traffic will arrive in two methods :
- From the Internet to the firewall
- From the Internet to the VPN gateway and then to the firewall
The advantage of this setup is that both type of clients (VPN and non-VPN) can be supported. All the clients will be
subjected to the firewall policy
The most advanced scenario introduces another firewall dedicated to control VPN traffic. From one box for all to dedicated boxes there is quite a lot of different options, from the cheapest to the most expensive. But as a general statement,
the best solution for one site is the solution conform to the security policy and where the pros and the cons of the solutions are well understood.
4.7 Implementation scenario
In order for two NMCs to establish a VPN link they must :
- confirm the protocols to be used (confirm use of tunnel mode, no encryption, SHA, pre-shared secrets)
- define the pres-shared secret. This “password” must be define and be the same on both sides
- confirm the VPN platform to be used
- agree on IP addresses to exchange on the link
- modify filter rules on the firewall. The following rules
o UDP port 500 is used for ISAKMP
o IP protocol number 50 (ESP protocol)
- implement the define configuration
- test
Once everything running, the main risk is the potential failure of the virtual link created.
CBS/ISS/VPN via the Internet, p. 20
5 Operators solutions
In the recent past years operator market for shared networks consist mainly in :
- Frame Relay for “slow” lines (up to few mb/s)
- ATM for higher speed communication.
These two solutions are layer 2 based, and therefore are completely independent of level 3 protocols. However, as IP is
the universal layer 3 protocol, technical alternatives solutions are offered.
The main disadvantage of Frame Relay and ATM are their static nature. The client must define exactly what peers are
allowed to exchange traffic and the throughput they want. These solutions are not flexible. It is costly and often create
administrative complexity o change the parameters and to allow a more dynamic approach.
5.1 Star, partially meshed, fully meshed networks
HeadQuarters
HeadQuarters
Site 4
Site 1
Site 4
Site 1
Site 3
Site 2
Site 3
Star network
Site 2
Either in Frame Relay or in ATM, the client must choose between :
- star network
- partially meshed
- fully meshed
HeadQuarters
Site 4
The choice is based on cost considerations (the price of these network is
often linked to numbers of PVCs and their speed) and traffic analysis.
It is often desirable to have more PVCs but thus increasing the cost of the
total network.
In the case of RMDCN for example it was decided to minimize the number of PVCs but, for less often communications, ECMWF offers Electronic Traffic Routing. Therefore, the total solution is a mixed between
operator service and self operated service.
Site 1
Site 3
Fully meshed
Site 2
Partially meshed
5.2 Technical solutions
CBS/ISS/VPN via the Internet, p. 21
Based on current infrastructure, Frame Relay and ATM, operators have new IP based services :
- MPLS VPN
- IPSec VPN
Of course, in both cases, just like with Frame Relay or ATM, the traffic from one client is isolated from the others.
Therefore, the client has a Virtual Private Network.
5.2.1 MPLS
MPLS is seen by the operators as a way to propose to their clients high speed communication in a more manageable way
compare to traditional solution. Based on their current Frame Relay or ATM network, MPLS is an easier way to propose
a partially meshed or a fully meshed network, and therefore, the network should be cheaper.
First experiences in Europe, however, show that the cost in not so different than with previous solutions !!
Some others services, like priorities might be proposed on MPLS networks.
5.2.2 IPSec VPN
The Internet is a collection of interconnected IPSs networks. Each ISP owns its infrastructure and interconnects with
others to provide Internet access to its clients.
Within its network, an ISP can guarantee a level of service and therefore, with add-on can propose to build a VPN based
on IPSec. In this case, VPN is a value added service on top of Internet connection.
But, as seen above in IPSec section, every connection must be manually created.
5.3 Conclusion
For the MTN or regional network, operators solutions offer guarantee of service, guaranteed bandwidth. But whatever
the technology the problems stay the same :
- the cost : an MPLS network is more expensive than a star Frame Relay solution (but probably cheaper than
fully meshed)
- the same operator must be available on every site.
- the problem of administrative cost sharing remain the same
The underlying technology for operators services is important but, this is not the first issue to consider.
CBS/ISS/VPN via the Internet, p. 22
6 Appendices
6.1 Example:A Cisco based configuration
6.1.1 The test platform
This test platform aims to demonstrate the connection of two Cisco routers using IPSEC over the Internet.
Serveur
Ethernet/IP
An IPSEC VPN over the Internet
ADSL
ATM25
Cisco 2611
Internet
Modem
VPN
ISDN
Cisco 1720
IBM Compatible
131.1.x.x
130.1.x.x
Both sites are connected to the Internet. The left one (see above) is connected through a permanent ADSL connection.
The right one connects through ISDN.
Both site musts keep normal Internet access. Internet accesses are not dedicated to IPSEC connection, and depending on
destination the connection must be direct or use the ad-hoc VPN.
ADSL site must have a permanent IP address on the ADSL side of the router. ISDN site receive a dynamic IP address
for every new connection. NAT (Network Address Translation) is used to masquerade internal IP addresses.
In paragraph 6.1.4 the full configuration are shown with an analysis of the main configuration topics.
For a full Cisco to-do list :
http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/secur_c/scprt4/scipsec.htm
6.1.2 ADSL connection with NAT
In order to understand the step-by-step connection mechanism we first indicate the ADSL connexion including NAT. In
this case, there is no IPSEC.
!
version 12.1
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname ADSL
!
enable secret 5 $1$zuPR$fsH7hiZKiW3ePfCXlBAAg.
enable password PWD
!
username UserNetissimo password 0 PwdNetissimo
!
memory-size iomem 15
ip subnet-zero
no ip finger
!
!
interface Ethernet0/0
ip address 130.1.8.1 255.255.0.0
ip nat inside
!
interface Ethernet0/1
no ip address
CBS/ISS/VPN via the Internet, p. 23
shutdown
!
interface ATM1/0
no ip address
atm vc-per-vp 4096
no atm ilmi-keepalive
pvc 2/32
encapsulation aal5mux ppp Virtual-Template1
!
interface Virtual-Template1
ip address negotiated
no ip redirects
no ip proxy-arp
ip nat outside
pulse-time 0
ppp chap hostname UserNetissimo
ppp chap password 7 00574404035D1207
no ppp chap wait
ppp pap sent-username UserNetissimo password 7 04085C040827554F
!
ip nat pool NET 193.253.191.14 193.253.191.14 netmask 255.255.255.0
ip nat inside source list 1 pool NET overload
ip classless
ip route 0.0.0.0 0.0.0.0 193.253.191.1
ip http server
!
access-list 1 permit any
!
line con 0
transport input none
line aux 0
line vty 0 4
password PWD
login
!
no scheduler allocate
end
6.1.3 VPN between the routers
ADSL Router :
!
version 12.1
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname ADSL
!
enable secret 5 $1$zuPR$fsH7hiZKiW3ePfCXlBAAg.
enable password PWD
!
username UserNetissimo password 0 PwdNetissimo
!
memory-size iomem 15
ip subnet-zero
no ip finger
!
crypto isakmp policy 1
authentication pre-share
crypto isakmp key IPSECKEY address 0.0.0.0
!
crypto ipsec transform-set CMT esp-null esp-sha-hmac
!
crypto dynamic-map MYMAP 10
set security-association lifetime seconds 600
set transform-set CMT
match address 100
!
crypto map CMMYMAP 30 ipsec-isakmp dynamic MYMAP discover
!
interface Ethernet0/0
ip address 130.1.8.1 255.255.0.0
!
interface Ethernet0/1
no ip address
shutdown
!
interface ATM1/0
CBS/ISS/VPN via the Internet, p. 24
no ip address
atm vc-per-vp 4096
no atm ilmi-keepalive
pvc 2/32
encapsulation aal5mux ppp Virtual-Template1
!
interface Virtual-Template1
ip address negotiated
no ip redirects
no ip proxy-arp
pulse-time 0
ppp chap hostname UserNetissimo
ppp chap password 7 00574404035D1207
no ppp chap wait
ppp pap sent-username UserNetissimo password 7 04085C040827554F
crypto map CMMYMAP
!
ip classless
ip route 0.0.0.0 0.0.0.0 193.253.191.1
ip http server
!
access-list 1 permit any
access-list 100 permit ip 130.1.0.0 0.0.255.255 131.1.0.0 0.0.255.255
!
line con 0
transport input none
line aux 0
line vty 0 4
password PWD
login
!
end
ISDN Router :
version 12.1
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname ISDN
!
enable secret 5 $1$n6eZ$UXI24gGl7lZu/wB3OZVnC1
enable password PWD
!
memory-size iomem 25
ip subnet-zero
no ip finger
no ip domain-lookup
!
isdn switch-type vn3
!
crypto isakmp policy 1
authentication pre-share
crypto isakmp key IPSECKEY address 193.253.191.14
!
crypto ipsec transform-set CMT esp-null esp-sha-hmac
!
crypto map CMYMAP 1 ipsec-isakmp
set peer 193.253.191.14
set security-association lifetime seconds 600
set transform-set CMT
match address 100
!
cns event-service server
!
interface BRI0
no ip address
encapsulation ppp
no ip mroute-cache
dialer pool-member 1
isdn switch-type vn3
isdn send-alerting
no cdp enable
crypto map CMYMAP
!
interface FastEthernet0
ip address 131.1.8.1 255.255.0.0
no ip mroute-cache
speed auto
full-duplex
CBS/ISS/VPN via the Internet, p. 25
!
interface Dialer0
ip address negotiated
encapsulation ppp
dialer remote-name LibertySurf
dialer pool 1
dialer string 0860155555
dialer-group 1
pulse-time 0
no cdp enable
ppp authentication chap pap callin
ppp chap hostname [email protected]
ppp chap password 7 1307160B04020A2F
ppp pap sent-username [email protected] password 7 14151312030A242E
crypto map CMYMAP
!
ip classless
ip route 0.0.0.0 0.0.0.0 Dialer0
no ip http server
!
access-list 1 permit any
access-list 100 permit ip 131.1.0.0 0.0.255.255 130.1.0.0 0.0.255.255
dialer-list 1 protocol ip permit
!
line con 0
transport input none
line aux 0
line vty 0 4
password PWD
login
!
no scheduler allocate
end
In the two configurations the red and underlines lines show the IPSEC set-up. Let’s focuses on this part of the configuration.
ADSL
crypto isakmp policy 1
authentication pre-share
crypto isakmp key IPSECKEY address 0.0.0.0
!
crypto ipsec transform-set CMT esp-null esp-sha-hmac
!
crypto dynamic-map MYMAP 10
set security-association lifetime seconds 600
set transform-set CMT
match address 100
!
crypto map CMMYMAP 30 ipsec-isakmp dynamic MYMAP discover
!
access-list 100 permit ip 130.1.0.0 0.0.255.255 131.1.0.0 0.0.255.255
!
interface ATM1/0
crypto map CMMYMAP
!
ISDN
crypto isakmp policy 1
authentication pre-share
crypto isakmp key IPSECKEY address 193.253.191.14
!
crypto ipsec transform-set CMT esp-null esp-sha-hmac
!
crypto map CMYMAP 1 ipsec-isakmp
set peer 193.253.191.14
set security-association lifetime seconds 600
set transform-set CMT
match address 100
!
access-list 100 permit ip 131.1.0.0 0.0.255.255 130.1.0.0 0.0.255.255
!
interface Dialer0
crypto map CMYMAP
6.1.3.1 IKE
In the configuration, this is related to the lines :
crypto isakmp policy 1
authentication pre-share
crypto isakmp key IPSECKEY address 0.0.0.0
The protocol used for IKA is isakmp and it has been established a pre-shared secret. This secret will be used by both
router to exchanges real encryption key.
IPSECKEY is the secret shared between the two routers.
On ADSL side, the originate IP address of the ISDN router is not known (in dialup connection like ISDN this address
will change for every new connection) therefore at this stage the 0.0.0.0 mean accept this IPSECKEY for every address.
This may lead to potential security risk. We will see below how Cisco permit to minimize it.
CBS/ISS/VPN via the Internet, p. 26
6.1.3.2 AH et ESP
The Authentication header protocol and the encapsulation security payload is then selected. Depending on the hashing
protocol available, the encryption solution, the choice to rely on ESP for both authentication and encryption or to AH,
the line :
crypto ipsec transform-set CMT esp-null esp-sha-hmac
describes which protocols are to be used for this particular IPSEC VPN. In our case, ESP is the preferred method for
both authentication and encryption.
ESP-NULL means use NULL for ESP (this is no encryption). ESP-SHA-HMAC means use SHA-HMAC for authentication with ESP. Of course the choice must be the same for both sides of the tunnel.
6.1.3.3 Security Association
The first two parts show the protocol part, let’s now work on Security Association. The ADSL must accept incoming
IPSEC connection from unknown peer :
crypto dynamic-map MYMAP 10
set security-association lifetime seconds 600
set transform-set CMT
match address 100
!
crypto map CMMYMAP 30 ipsec-isakmp dynamic MYMAP discover
!
access-list 100 permit ip 130.1.0.0 0.0.255.255 131.1.0.0 0.0.255.255
Therefore, Cisco offers the “dynamic-map” solution. In this case, the ADSL router will accept every new connection using the right protocols (esp-null and esp-sha-hmac) from any IP address using the right password IPSECKEY. In order to restrict
the use of this IPSEC tunnel Cisco use the mecanism of access-list. The access-list 100 must be the same on both side of the tunnel
in order to allow the IPSEC connection. This is a Cisco dependant feature.
6.1.4 Final configuration
A mixed of above configuration NAT and IPSEC are gathered in one configuration.
Comments in red help the reader to identify the parts in the configuration, which are site dependant. This includes:
- IP addresses on both sites
- Username and password used to connect to the ISPs (Internet Service Provider)
- Access-lists
- IPSEC parameters
This configuration can be used as an example or a “to-do list” when configuring IPSEC between two sites.
ADSL Router :
!
version 12.1
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname ADSL
!
enable secret 5 $1$zuPR$fsH7hiZKiW3ePfCXlBAAg.
enable password PWD
!
username UserNetissimo password 0 PwdNetissimo
!
memory-size iomem 15
ip subnet-zero
no ip finger
!
crypto isakmp policy 1
authentication pre-share
crypto isakmp key IPSECKEY address 0.0.0.0
!
crypto ipsec transform-set CMT esp-null esp-sha-hmac
!
crypto dynamic-map MYMAP 10
set security-association lifetime seconds 600
set transform-set CMT
match address 100
username and password used to connect
to the internet through ADSL. This
depend on agreement with local service
provider (ISP)
CBS/ISS/VPN via the Internet, p. 27
!
crypto map CMMYMAP 30 ipsec-isakmp dynamic MYMAP discover
!
interface Ethernet0/0
ip address 130.1.8.1 255.255.0.0
IP address of local site
ip nat inside
!
interface Ethernet0/1
no ip address
shutdown
!
interface ATM1/0
ATM interface for ADSL link
no ip address
These parameters are ISP dependant
atm vc-per-vp 4096
no atm ilmi-keepalive
pvc 2/32
encapsulation aal5mux ppp Virtual-Template1
!
interface Virtual-Template1
Interface for virtual link on top
ip address negotiated
of ADSL connection. All the PPP
no ip redirects
settings are site dependant
no ip proxy-arp
ip nat outside
pulse-time 0
ppp chap hostname UserNetissimo
ppp chap password 7 00574404035D1207
no ppp chap wait
ppp pap sent-username UserNetissimo password 7 04085C040827554F
crypto map CMMYMAP
!
ip nat pool NET 193.253.191.14 193.253.191.14 netmask 255.255.255.0
ip nat inside source list 101 pool NET overload
The IP address 193.253.191.14 is
ip classless
the IP address given by the ISP
ip route 0.0.0.0 0.0.0.0 193.253.191.1
It is used to translate internal
ip http server
private addresses to IP addresses
!
in conformance to the Internet rules
access-list 1 permit any
access-list 100 permit ip 130.1.0.0 0.0.255.255 131.1.0.0 0.0.255.255
access-list 101 deny
ip 130.1.0.0 0.0.255.255 131.1.0.0 0.0.255.255
access-list 101 permit ip any any
See comments on ACL above
!
line con 0
transport input none
line aux 0
line vty 0 4
password PWD
login
!
end
ISDN Router :
!
version 12.1
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname ISDN
!
enable secret 5 $1$n6eZ$UXI24gGl7lZu/wB3OZVnC1
enable password PWD
!
memory-size iomem 25
ip subnet-zero
no ip finger
no ip domain-lookup
!
isdn switch-type vn3
!
crypto isakmp policy 1
authentication pre-share
crypto isakmp key CléIPSEC address 193.253.191.14
!
crypto ipsec transform-set CMT esp-null esp-sha-hmac
!
crypto map CMYMAP 1 ipsec-isakmp
set peer 193.253.191.14
set security-association lifetime seconds 600
set transform-set CMT
match address 100
This IP address is the “other end”
of the VPN tunnel. It depends on the
local situation
CBS/ISS/VPN via the Internet, p. 28
!
cns event-service server
!
interface BRI0
no ip address
ip nat outside
encapsulation ppp
no ip mroute-cache
dialer pool-member 1
isdn switch-type vn3
isdn send-alerting
no cdp enable
crypto map CMYMAP
!
interface FastEthernet0
ip address 131.1.8.1 255.255.0.0
IP address of local site
ip nat inside
no ip mroute-cache
speed auto
full-duplex
!
interface Dialer0
The two dialers below are ISP
ip address negotiated
dependant : phone number, username
encapsulation ppp
and password
dialer remote-name LibertySurf
dialer pool 1
dialer string 0860155555
dialer-group 1
pulse-time 0
no cdp enable
ppp authentication chap pap callin
ppp chap hostname [email protected]
ppp chap password 7 1307160B04020A2F
ppp pap sent-username [email protected] password 7 14151312030A242E
crypto map CMYMAP
!
interface Dialer1
ip address negotiated
ip nat outside
encapsulation ppp
dialer pool 1
dialer string 0860155555
dialer-group 1
pulse-time 0
no cdp enable
ppp authentication chap pap callin
ppp chap hostname [email protected]
ppp chap password 7 1307160B04020A2F
ppp pap sent-username [email protected] password 7 14151312030A242E
!
ip nat inside source list 101 interface Dialer1 overload
ip classless
ip route 0.0.0.0 0.0.0.0 Dialer1
ip route 130.1.0.0 255.255.0.0 Dialer0
ip route 193.253.191.14 255.255.255.255 Dialer0
no ip http server
!
access-list 1 permit any
access-list 100 permit ip 131.1.0.0 0.0.255.255 130.1.0.0 0.0.255.255
access-list 101 deny
ip 131.1.0.0 0.0.255.255 130.1.0.0 0.0.255.255
access-list 101 permit ip any any
See comments on ACL above
dialer-list 1 protocol ip permit
!
line con 0
transport input none
line aux 0
line vty 0 4
password PWD
login
!
end
6.1.5 Performance
The categorization of Cisco routers is a good start for many situations. However, it can be quite tricky when it involves
building a complex and/or large VPN solution. The deployment of the right router at the right place will contribute significantly to the overall VPN design in term of performance as well as cost.
CBS/ISS/VPN via the Internet, p. 29
It is important to note that additional Cisco IOS processing will be required for all packets that require security services.
The IPSec technology has shown to be slower when compared with Cisco Encryption Technology (CET). One of the
reasons is because of the deployment of authentication algorithms which are considered to be slow.
Nonetheless, security literature has highlighted that lack of authentication is a security vulnerability.
Without compromising security, IPSec must be the technology to deploy. In addition, IPSec introduces packet expansion, which is more likely to require fragmentation and reassembly of IPSec-protected IP datagrams. The encrypted
packets will also be authenticated which means that most packets will have two cryptographic operations performed on
them.
Model
Baseline
Authentication
only
Encryption
only
Authentication
and encryption
Suggested
bandwidth
Cisco 2514
2.4-9.9
0.1-1.0
0.16-0.25
0.1-0.2
Up to 128kbps
Cisco 3640
9.9+
0.6-6.1
0.7-2.5
0.5-2.1
Up to 1.5Mbps
Cisco 7206
9.9+
1.0-9.1
1.1-3.5
0.9-2.9
Up to 2.5Mbps
Cisco 7505
9.9+
2.9-9.4
3.6-9.1
2.6-7.9
Up to 6Mbps
Performance ranges in Mbps
The minimum number is obtained with packet size of 64 bytes, the maximum with 1024 bytes. For example with a Cisco
3640 and packets of 64 bytes, the maximum bandwidth is 500kbps. When this number is compared to the baseline information, it means that the performance with encryption and authentication of the router is only 5% of the “normal”
performance.
6.2 Glossary
The following glossary is based on http://www.freeswan.org/freeswan_trees/freeswan-1.95/doc/glossary.html
3DES (Triple DES)
Using three DES encryptions on a single data block, with at least two different keys, to get higher security than is
available from a single DES pass.
IPsec always does 3DES with three different keys, as required by RFC 2451. For an explanation of the two-key
variant, see two key triple DES. Both use an EDE encrypt-decrypt-encrpyt sequence of operations.
Single DES is insecure.
Double DES is ineffective. Using two 56-bit keys, one might expect an attacker to have to do 2112 work to break
it. In fact, only 257 work is required with a meet-in-the-middle attack, though a large amount of memory is also
required. Triple DES is vulnerable to a similar attack, but that just reduces the work factor from the 2168 one
might expect to 2 112. That provides adequate protection against brute force attacks, and no better attack is
known.
3DES can be somewhat slow compared to other ciphers. It requires three DES encryptions per block. DES was
designed for hardware implementation and includes some operations which are difficult in software. However,
the speed we get is quite acceptable for many uses. See our performance document for details.
AES
The Advanced Encryption Standard, a new block cipher standard to replace DES being developed by NIST, the
US National Institute of Standards and Technology. DES used 64-bit blocks and a 56-bit key. AES ciphers use a
128-bit block and are required to support 128, 192 and 256-bit keys. Some of them support other sizes as well.
The larger block size helps resist birthday attacks while the large key size prevents brute force attacks.
Fifteen proposals meeting NIST's basic criteria were submitted in 1998 and subjected to intense discussion and
analysis, "round one" evaluation. In August 1999, NIST narrowed the field to five "round two" candidates:
· Mars from IBM
· RC6 from RSA
· Rijndael from two Belgian researchers
· Serpent, a British-Norwegian-Israeli research collaboration
CBS/ISS/VPN via the Internet, p. 30
· Twofish from the consulting firm Counterpane
Three of the five finalists -- Rijndael, Serpent and Twofish -- have completely open licenses.
In October 2000, NIST announced the winner -- Rijndael.
For more information, see:
· NIST's AES home page
· the Block Cipher Lounge AES page
· Brian Gladman's code and benchmarks
· Helger Lipmaa's survey of implementations
Adding one or more AES ciphers to Linux FreeS/WAN would be a useful undertaking. Likely one would add all
three of the Round Two candidates with good licenses. A complication is that our code is built for a 64-bit block
cipher and AES uses a 128-bit block. Volunteers via the mailing lists would be welcome.
AH
The IPsec Authentication Header, added after the IP header. For details, see IPsec document and/or RFC 2402.
Asymmetric cryptography
See public key cryptography.
Authentication
Ensuring that a message originated from the expected sender and has not been altered on route. IPsec uses authentication in two places:
· peer authentication, authenticating the players in IKE's Diffie-Hellman key exchanges to prevent man-in-themiddle attacks. This can be done in a number of ways. The methods supported by FreeS/WAN are discussed in
our configuration document.
· packet authentication, authenticating packets on an established SA, either with a separate authentication header
or with the optional authentication in the ESP protocol. In either case, packet authentication uses a hashed message athentication code technique.
Outside IPsec, passwords are perhaps the most common authentication mechanism. Their function is essentially
to authenticate the person's identity to the system. Passwords are generally only as secure as the network they
travel over. If you send a cleartext password over a tapped phone line or over a network with a packet sniffer on
it, the security provided by that password becomes zero. Sending an encrypted password is no better; the attacker
merely records it and reuses it at his convenience. This is called a replay attack.
A common solution to this problem is a challenge-response system. This defeats simple eavesdropping and replay attacks. Of course an attacker might still try to break the cryptographic algorithm used, or the random number generator.
Birthday attack
A cryptographic attack based on the mathematics exemplified by the birthday paradox. This math turns up whenever the question of two cryptographic operations producing the same result becomes an issue:
· collisions in message digest functions.
· identical output blocks from a block cipher
· repetition of a challenge in a challenge-response system
Resisting such attacks is part of the motivation for:
· hash algorithms such as SHA and RIPEMD-160 giving a 160-bit result rather than the 128 bits of MD4, MD5
and RIPEMD-128.
· AES block ciphers using a 128-bit block instead of the 64-bit block of most current ciphers
· IPsec using a 32-bit counter for packets sent on an automatically keyed SA and requiring that the connection
always be rekeyed before the counter overflows.
Birthday paradox
Not really a paradox, just a rather counter-intuitive mathematical fact. In a group of 23 people, the chance of a
least one pair having the same birthday is over 50%.
The second person has 1 chance in 365 (ignoring leap years) of matching the first. If they don't match, the third
person's chances of matching one of them are 2/365. The 4th, 3/365, and so on. The total of these chances grows
more quickly than one might guess.
Block cipher
A symmetric cipher which operates on fixed-size blocks of plaintext, giving a block of ciphertext for each. Contrast with stream cipher. Block ciphers can be used in various modes when multiple block are to be encrypted.
CBS/ISS/VPN via the Internet, p. 31
DES is among the the best known and widely used block ciphers, but is now obsolete. Its 56-bit key size makes it
highly insecure today.
The current generation of block ciphers -- such as Blowfish, CAST-128 and IDEA -- all use 64-bit blocks and
128-bit keys. The next generation, AES, uses 128-bit blocks and supports key sizes up to 256 bits.
The Block Cipher Lounge web site has more information.
Blowfish
A block cipher using 64-bit blocks and keys of up to 448 bits, designed by Bruce Schneier and used in several
products.
This is not required by the IPsec RFCs
Brute force attack (exhaustive search)
Breaking a cipher by trying all possible keys. This is always possible in theory (except against a one-time pad),
but it becomes practical only if the key size is inadequate. For an important example, see our document on the insecurity of DES with its 56-bit key. For an analysis of key sizes required to resist plausible brute force attacks,
see this paper.
Longer keys protect against brute force attacks. Each extra bit in the key doubles the number of possible keys and
therefore doubles the work a brute force attack must do. A large enough key defeats any brute force attack.
For example, the EFF's DES Cracker searches a 56-bit key space in an average of a few days. Let us assume an
attacker that can find a 64-bit key (256 times harder) by brute force search in a second (a few hundred thousand
times faster). For a 96-bit key, that attacker needs 232 seconds, about 135 years. Against a 128-bit key, he needs
232 times that, over 500,000,000,000 years. Your data is then obviously secure against brute force attacks. Even
if our estimate of the attacker's speed is off by a factor of a million, it still takes him over 500,000 years to crack
a message.
This is why
· single DES is now considered dangerously insecure
· all of the current generation of block ciphers use a 128-bit or longer key
· AES ciphers support keysizes 128, 192 and 256 bits
· any cipher we add to Linux FreeS/WAN will have at least a 128-bit key
Cautions:
Inadequate keylength always indicates a weak cipher but it is important to note that adequate keylength does not
necessarily indicate a strong cipher. There are many attacks other than brute force, and adequate keylength only
guarantees resistance to brute force. Any cipher, whatever its key size, will be weak if design or implementation
flaws allow other attacks.
Also, once you have adequate keylength (somewhere around 90 or 100 bits), adding more key bits make no practical difference , even against brute force. Consider our 128-bit example above that takes 500,000,000,000 years
to break by brute force. We really don't care how many zeroes there are on the end of that, as long as the number
remains ridiculously large. That is, we don't care exactly how large the key is as long as it is large enough.
There may be reasons of convenience in the design of the cipher to support larger keys. For example Blowfish allows up to 448 bits and RC4 up to 2048, but beyond 100-odd bits it makes no difference to practical security.
CBC mode
Cipher Block Chaining mode , a method of using a block cipher in which for each block except the first, the result of the previous encryption is XORed into the new block before it is encrypted. CBC is the mode used in IPsec.
An initialisation vector (IV) must be provided. It is XORed into the first block before encryption. The IV need
not be secret but should be different for each message and unpredictable.
Cipher Modes
Different ways of using a block cipher when encrypting multiple blocks.
Four standard modes were defined for DES in FIPS 81. They can actually be applied with any block cipher.
ECB
Electronic CodeBook
encrypt each block independently
CBC
Cipher Block Chaining
XOR previous block ciphertext into new block plaintext before encrypting
new block
CFB
Cipher FeedBack
OFB
Output FeedBack
IPsec uses CBC mode since this is only marginally slower than ECB and is more secure. In ECB mode the same
plaintext always encrypts to the same ciphertext, unless the key is changed. In CBC mode, this does not occur.
Various other modes are also possible, but none of them are used in IPsec.
CBS/ISS/VPN via the Internet, p. 32
Ciphertext
The encrypted output of a cipher, as opposed to the unencrypted plaintext input.
Client
This term has at least two distinct uses in discussing IPsec:
· The clients of an IPsec gateway are the machines it protects, typically on one or more subnets behind the gateway. In this usage, all the machines on an office network are clients of that office's IPsec gateway. Laptop or
home machines connecting to the office, however, are not clients of that gateway. They are remote gateways,
running the other end of an IPsec connection. Each of them is also its own client.
· IPsec client software is used to describe software which runs on various standalone machines to let them connect to IPsec networks. In this usage, a laptop or home machine connecting to the office is a client machine.
We generally use the term in the first sense. Vendors of Windows IPsec solutions often use it in the second.
Denial of service (DoS) attack
An attack that aims at denying some service to legitimate users of a system, rather than providing a service to the
attacker.
· One variant is a flooding attack, overwhelming the system with too many packets, to much email, or whatever.
· A closely related variant is a resource exhaustion attack. For example, consider a "TCP SYN flood" attack. Setting up a TCP connection involves a three-packet exchange:
o Initiator: Connection please (SYN)
o Responder: OK (ACK)
o Initiator: OK here too
If the attacker puts bogus source information in the first packet, such that the second is never delivered, the responder may wait a long time for the third to come back. If responder has already allocated memory for the connection data structures, and if many of these bogus packets arrive, the responder may run out of memory.
· Another variant is to feed the system undigestible data, hoping to make it sick. For example, IP packets are limited in size to 64K bytes and a fragment carries information on where it starts within that 64K and how long it is.
The "ping of death" delivers fragments that say, for example, that they start at 60K and are 20K long. Attempting
to re-assemble these without checking for overflow can be fatal.
The two example attacks discussed were both quite effective when first discovered, capable of crashing or disabling many operating systems. They were also well-publicised, and today far fewer systems are vulnerable to
them.
DES
The Data Encryption Standard, a block cipher with 64-bit blocks and a 56-bit key. Probably the most widely used
symmetric cipher ever devised. DES has been a US government standard for their own use (only for unclassified
data), and for some regulated industries such as banking, since the late 70's.
DES is seriously insecure against current attacks.
DH
see Diffie-Hellman
Diffie-Hellman (DH) key exchange protocol
A protocol that allows two parties without any initial shared secret to create one in a manner immune to eavesdropping. Once they have done this, they can communicate privately by using that shared secret as a key for a
block cipher or as the basis for key exchange.
The protocol is secure against all passive attacks, but it is not at all resistant to active man-in-the-middle attacks.
If a third party can impersonate Bob to Alice and vice versa, then no useful secret can be created. Authentication
of the participants is a prerequisite for safe Diffie-Hellman key exchange. IPsec can use any of several authentication mechanisims. Those supported by FreeS/WAN are discussed in our configuration section.
The Diffie-Hellman key exchange is based on the discrete logarithm problem and is secure unless someone finds
an efficient solution to that problem.
Given a prime p and generator g (explained under discrete log below), Alice:
· generates a random number a
· calculates A = g^a modulo p
· sends A to Bob
Meanwhile Bob:
· generates a random number b
CBS/ISS/VPN via the Internet, p. 33
· calculates B = g^b modulo p
· sends B to Alice
Now Alice and Bob can both calculate the shared secret s = g^(ab). Alice knows a and B, so she calculates s =
B^a. Bob knows A and b so he calculates s = A^b.
An eavesdropper will know p and g since these are made public, and can intercept A and B but, short of solving
the discrete log problem, these do not let him or her discover the secret s.
Digital signature
Sender:
· calculates a message digest of a document
· encrypts the digest with his or her private key, using some public key cryptosystem.
· attaches the encrypted digest to the document as a signature
Receiver:
· calculates a digest of the document (not including the signature)
· decrypts the signature with the signer's public key
· verifies that the two results are identical
If the public-key system is secure and the verification succeeds, then the receiver knows
· that the document was not altered between signing and verification
· that the signer had access to the private key
Such an encrypted message digest can be treated as a signature since it cannot be created without both the document and the private key which only the sender should possess. The legal issues are complex, but several countries are moving in the direction of legal recognition for digital signatures.
EAR
The US government's Export Administration R egulations, administered by the Bureau of Export Administration.
These have replaced the earlier ITAR regulations as the controls on export of cryptography.
EDE
The sequence of operations normally used in either the three-key variant of triple DES used in IPsec or the twokey variant used in some other systems.
The sequence is:
· Encrypt with key1
· Decrypt with key2
· Encrypt with key3
For the two-key version, key1=key3.
The "advantage" of this EDE order of operations is that it makes it simple to interoperate with older devices offering only single DES. Set key1=key2=key3 and you have the worst of both worlds, the overhead of triple DES
with the security of single DES. Since single DES is insecure, this is an extremely dubious "advantage".
The EDE two-key variant can also interoperate with the EDE three-key variant used in IPsec; just set k1=k3.
Encryption
Techniques for converting a readable message ( plaintext) into apparently random material ( ciphertext) which
cannot be read if intercepted. A key is required to read the message.
Major variants include symmetric encryption in which sender and receiver use the same secret key and public key
methods in which the sender uses one of a matched pair of keys and the receiver uses the other. Many current
systems, including IPsec, are hybrids combining the two techniques.
ESP
Encapsulated Security Payload, the IPsec protocol which provides encryption. It can also provide authentication
service and may be used with null encryption (which we do not recommend). For details see our IPsec document
and/or RFC 2406.
FIPS
Federal Information Processing S tandard, the US government's standards for products it buys. These are issued
by NIST. Among other things, DES and SHA are defined in FIPS documents. NIST have a FIPS home page.
Hash
see message digest
CBS/ISS/VPN via the Internet, p. 34
Hashed Message Authentication Code (HMAC)
using keyed message digest functions to authenticate a message. This differs from other uses of these functions:
· In normal usage, the hash function's internal variable are initialised in some standard way. Anyone can reproduce the hash to check that the message has not been altered.
· For HMAC usage, you initialise the internal variables from the key. Only someone with the key can reproduce
the hash. A successful check of the hash indicates not only that the message is unchanged but also that the creator
knew the key.
The exact techniques used in IPsec are defined in RFC 2104. They are referred to as HMAC-MD5-96 and
HMAC-SHA-96 because they output only 96 bits of the hash. This makes some attacks on the hash functions
harder.
HMAC
see Hashed Message Authentication Code
HMAC-MD5-96
see Hashed Message Authentication Code
HMAC-SHA-96
see Hashed Message Authentication Code
ICMP
Internet Control M essage Protocol. This is used for various IP-connected devices to manage the network.
IDEA
International Data Encrypion Algorithm, developed in Europe as an alternative to exportable American ciphers
such as DES which were too weak for serious use. IDEA is a block cipher using 64-bit blocks and 128-bit keys,
and is used in products such as PGP.
IDEA is not required by the IPsec RFCs and not currently used in Linux FreeS/WAN.
IDEA is patented and, with strictly limited exceptions for personal use, using it requires a license from Ascom.
IKE
Internet Key Exchange, based on the Diffie-Hellman key exchange protocol.
Initialisation Vector (IV)
Some cipher modes, including the CBC mode which IPsec uses, require some extra data at the beginning. This
data is called the initialisation vector. It need not be secret, but should be different for each message. Its function
is to prevent messages which begin with the same text from encrypting to the same ciphertext. That might give an
analyst an opening, so it is best prevented.
IP
Internet Protocol.
IPSec
Internet Protocol SECurity, security functions (authentication and encryption) implemented at the IP level of the
protocol stack. It is optional for IPv4 and mandatory for IPv6.
ISAKMP
Internet Security Association and Key Management Protocol, defined in RFC 2408.
Man-in-the-middle attack
An active attack in which the attacker impersonates each of the legitimate players in a protocol to the other.
For example, if Alice and Bob are negotiating a key via the Diffie-Hellman key agreement, and are not using authentication to be certain they are talking to each other, then an attacker able to insert himself in the communication path can deceive both players.
Call the attacker Mallory. For Bob, he pretends to be Alice. For Alice, he pretends to be Bob. Two keys are then
negotiated, Alice-to-Mallory and Bob-to-Mallory. Alice and Bob each think the key they have is Alice-to-Bob.
CBS/ISS/VPN via the Internet, p. 35
A message from Alice to Bob then goes to Mallory who decrypts it, reads it and/or saves a copy, re-encrypts using the Bob-to-Mallory key and sends it along to Bob. Bob decrypts successfully and sends a reply which Mallory decrypts, reads, re-encrypts and forwards to Alice.
To make this attack effective, Mallory must
· subvert some part of the network in some way that lets him carry out the deception
possible targets: DNS, router, Alice or Bob's machine, mail server, ...
· beat any authentication mechanism Alice and Bob use
strong authentication defeats the attack entirely; this is why IKE requires authentication
· work in real time, delivering messages without introducing a delay large enough to alert the victims
not hard if Alice and Bob are using email; quite difficult in some situations.
If he manages it, however, it is devastating. He not only gets to read all the messages; he can alter messages, inject his own, forge anything he likes, . . . In fact, he controls the communication completely.
Manual keying
An IPsec mode in which the keys are provided by the administrator. The alternative, automatic keying, is preferred in most cases.
MD4
Message Digest Algorithm Four from Ron Rivest of RSA. MD4 was widely used a few years ago, but is now
considered obsolete. It has been replaced by its descendants MD5 and SHA.
MD5
Message Digest Algorithm Five from Ron Rivest of RSA, an improved variant of his MD4. Like MD4, it produces a 128-bit hash. For details see RFC 1321.
MD5 is one of two message digest algorithms available in IPsec. The other is SHA. SHA produces a longer hash
and is therefore more resistant to birthday attacks, but this is not a concern for IPsec. The HMAC method used in
IPsec is secure even if the underlying hash is not particularly strong against this attack.
Meet-in-the-middle attack
A divide-and-conquer attack which breaks a cipher into two parts, works against each separately, and compares
results. Probably the best known example is an attack on double DES. This applies in principle to any pair of
block ciphers, e.g. to an encryption system using, say, CAST-128 and Blowfish, but we will describe it for double DES.
Double DES encryption and decryption can be written:
C = E(k2,E(k1,P))
P = D(k1,D(k2,C))
Where C is ciphertext, P is plaintext, E is encryption, D is decryption, k1 is one key, and k2 is the other key. If
we know a P, C pair, we can try and find the keys with a brute force attack, trying all possible k1, k2 pairs. Since
each key is 56 bits, there are 2 112 such pairs and this attack is painfully inefficient.
The meet-in-the middle attack re-writes the equations to calculate a middle value M:
M = E(k1,P)
M = D(k2,C)
Now we can try some large number of D(k2,C) decryptions with various values of k2 and store the results in a table. Then start doing E(k1,P) encryptions, checking each result to see if it is in the table.
With enough table space, this breaks double DES with 256 + 256 = 257 work. Against triple DES, you need 256
+ 2112 ~= 2112.
The memory requirements for such attacks can be prohibitive, but there is a whole body of research literature on
methods of reducing them.
Message Digest Algorithm
An algorithm which takes a message as input and produces a hash or digest of it, a fixed-length set of bits which
depend on the message contents in some highly complex manner. Design criteria include making it extremely difficult for anyone to counterfeit a digest or to change a message without altering its digest. One essential property
is collision resistance. The main applications are in message authentication and digital signature schemes. Widely
used algorithms include MD5 and SHA. In IPsec, message digests are used for HMAC authentication of packets.
NIST
The US National Institute of Standards and Technology, responsible for FIPS standards including DES and its
replacement, AES.
Non-routable IP address
CBS/ISS/VPN via the Internet, p. 36
An IP address not normally allowed in the "to" or "from" IP address field header of IP packets.
Almost invariably, the phrase "non-routable address" means one of the addresses reserved by RFC 1918 for private networks:
· 10.anything
· 172.x.anything with 16 <= x <= 31
· 192.168.anything
These addresses are commonly used on private networks, e.g. behind a Linux machines doing IP masquerade.
Machines within the private network can address each other with these addresses. All packets going outside that
network, however, have these addresses replaced before they reach the Internet.
If any packets using these addresses do leak out, they do not go far. Most routers automatically discard all such
packets.
Various other addresses -- the 127.0.0.0/8 block reserved for local use, 0.0.0.0, various broadcast and network
addresses -- cannot be routed over the Internet, but are not normally included in the meaning when the phrase
"non-routable address" is used.
Oakley
A key determination protocol, defined in RFC 2412.
Oakley groups
The groups used as the basis of Diffie-Hellman key exchange in the Oakley protocol, and in IKE. Four were defined in the original RFC, and a fifth has been added since.
One time pad
A cipher in which the key is:
· as long as the total set of messages to be enciphered
· absolutely random
· never re-used
Given those three conditions, it can easily be proved that the cipher is perfectly secure, in the sense that an attacker with intercepted message in hand has no better chance of guessing the message than an attacker who has
not intercepted the message and only knows the message length. No such proof exists for any other cipher.
There are, however, several problems with this "perfect" cipher.
First, it is wildly impractical for most applications. Key management is at best difficult, often completely impossible.
Second, it is extremely fragile. Small changes which violate the conditions listed above do not just weaken the
cipher a bit; quite often they destroy its security completely.
· Re-using the pad weakens it to the point where it can be broken with pencil and paper. With a computer, the attack is trivially easy.
· Using computer-generated pseudo-random numbers instead of a really random pad completely invalidates the
security proof. Depending on random number generator used, this may also give an extremely weak cipher.
Marketing claims about the "unbreakable" security of various products which somewhat resemble one-time pads
are common. Such claims are one of the surest signs of cryptographic snake oil. Systems marketed with such
claims are usually completely worthless.
Finally, even if the system is implemented and used correctly, it is highly vulnerable to certain types of attack. If
an attacker knows the plaintext and has an intercepted message, he can discover the pad. This does not matter if
the attacker is just a passive eavesdropper. It gives him no plaintext he didn't already know and we don't care that
he learns a pad which we'll never re-use. However, knowing the pad lets an active attacker perform a man-in-themiddle attack, replacing your message with whatever he chooses.
Photuris
Another key negotiation protocol, an alternative to IKE, described in RFCs 2522 and 2523.
PPTP
Point-to-Point Tunneling Protocol. Papers discussing weaknesses in it are on counterpane.com.
PKI
Public Key Infrastructure, the things an organisation or community needs to set up in order to make public key
cryptographic technology a standard part of their operating procedures.
CBS/ISS/VPN via the Internet, p. 37
There are several PKI products on the market. Typically they use a hierarchy of Certification Authorities (CAs).
Often they use LDAP access to X.509 directories to implement this.
Plaintext
The unencrypted input to a cipher, as opposed to the encrypted ciphertext output.
Public Key Cryptography
In public key cryptography, keys are created in matched pairs. Encrypt with one half of a pair and only the
matching other half can decrypt it. This contrasts with symmetric or secret key cryptography in which a single
key known to both parties is used for both encryption and decryption.
One half of each pair, called the public key, is made public. The other half, called the private key, is kept secret.
Messages can then be sent by anyone who knows the public key to the holder of the private key. Encrypt with the
public key and you know only someone with the matching private key can decrypt.
Public key techniques can be used to create digital signatures and to deal with key management issues, perhaps
the hardest part of effective deployment of symmetric ciphers. The resulting hybrid cryptosystems use public key
methods to manage keys for symmetric ciphers.
Many organisations are currently creating PKIs, public key infrastructures to make these benefits widely available.
Public Key Infrastructure
see PKI
RC4
Rivest Cipher four, designed by Ron Rivest of RSA and widely used. Believed highly secure with adequate key
length, but often implemented with inadequate key length to comply with export restrictions.
RC6
Rivest Cipher six, RSA's AES candidate cipher.
Replay attack
An attack in which the attacker records data and later replays it in an attempt to deceive the recipient.
RFC
Request For Comments, an Internet document. Some RFCs are just informative
Routable IP address
Most IP addresses can be used as "to" and "from" addresses in packet headers. These are the routable addresses;
we expect routing to be possible for them. If we send a packet to one of them, we expect (in most cases; there are
various complications) that it will be delivered if the address is in use and will cause an ICMP error packet to
come back to us if not.
There are also several classes of non-routable IP addresses.
RSA algorithm
Rivest Shamir Adleman public key encryption method, named for its three inventors. The algorithm is widely
used and likely to become moreso since it became free of patent encumbrances in September 2000.
For a full explanation of the algorithm, consult Applied Cryptography from B.Schneier. A simple explanation is:
The great 17th century French mathematician Fermat proved that, for any prime p and number x, 0 x^p == x
modulo p x^(p-1) == 1 modulo p, non-zero x From this it follows that if we have a pair of primes p, q and two
numbers e, d such that:
ed == 1
modulo lcm( p-1, q-1)
where lcm() is least common multiple, then for all x, 0 x^(ed-1) == 1 modulo pq, non-zero x x^ed == x modulo
pq So we construct such as set of numbers p, q, e, d and publish the product N=pq and e as the public key. Encryption is then:
c = x^e
modulo N
An attacker cannot deduce x from the cyphertext c, short of either factoring N or solving the discrete logarithm
problem for this field. If p, q are large primes (hundreds or thousands of bits) no efficient solution to either problem is known.
The receiver, knowing the private key (N and d), can readily find x sixce:
c^d == (x^e)^d
modulo N
CBS/ISS/VPN via the Internet, p. 38
== x^ed
modulo N
== x
modulo N
This gives an effective public key technique, with only a couple of problems. It uses a good deal of computer
time, since calculations with large integers are not cheap, and there is no proof it is necessarily secure since noone has proven either factoring or discrete log cannot be done efficiently.
RSA Data Security
A company founded by the inventors of the RSA public key algorithm.
SA
Security Association, the channel negotiated by the higher levels of an IPsec implementation and used by the
lower. SAs are unidirectional; you need a pair of them for two-way communication.
An SA is defined by three things -- the destination, the protocol (AH orESP) and the SPI , security parameters
index. It is used to index other things such as session keys and initialization vectors.
For more detail, see our section on IPsec and/or RFC 2401.
Security Association
see SA
SHA
Secure Hash Algorithm, a message digest algorithm developed by the NSA for use in the Digital Signature standard, FIPS number 186 from NIST. SHA is an improved variant of MD4 producing a 160-bit hash.
SHA is one of two message digest algorithms available in IPsec. The other is MD5. Some people do not trust
SHA because it was developed by the NSA. There is, as far as we know, no cryptographic evidence that SHA is
untrustworthy, but this does not prevent that view from being strongly held.
SKIP
Simple Key management for Internet P rotocols, an alternative to IKE developed by Sun and being marketed by
their Internet Commerce Group.
SPI
Security Parameter Index, an index used within IPsec to keep connections distinct. A Security Association (SA)
is defined by destination, protocol and SPI. Without the SPI, two connections to the same gateway using the
same protocol could not be distinguished.
For more detail, see our IPsec section and/or RFC 2401.
SSH
Secure SHell, an encrypting replacement for the insecure Berkeley commands whose names begin with "r" for
"remote": rsh, rlogin, etc.
SSL
Secure Sockets Layer , a set of encryption and authentication services for web browsers, developed by Netscape.
Widely used in Internet commerce. Also known as TLS.
Symmetric cryptography
Symmetric cryptography, also referred to as conventional or secret key cryptography, relies on a shared secret
key, identical for sender and receiver. Sender encrypts with that key, receiver decrypts with it. The idea is that an
eavesdropper without the key be unable to read the messages. There are two main types of symmetric cipher,
block ciphers and stream ciphers.
Symmetric cryptography contrasts with public key or asymmetric systems where the two players use different
keys.
The great difficulty in symmetric cryptography is, of course, key management. Sender and receiver must have
identical keys and those keys must be kept secret from everyone else. Not too much of a problem if only two
people are involved and they can conveniently meet privately or employ a trusted courier. Quite a problem,
though, in other circumstances.
It gets much worse if there are many people. An application might be written to use only one key for communication among 100 people, for example, but there would be serious problems. Do you actually trust all of them that
CBS/ISS/VPN via the Internet, p. 39
much? Do they trust each other that much? Should they? What is at risk if that key is compromised? How are you
going to distribute that key to everyone without risking its secrecy? What do you do when one of them leaves the
company? Will you even know?
On the other hand, if you need unique keys for every possible connection between a group of 100, then each user
must have 99 keys. You need either 99*100/2 = 4950 secure key exchanges between users or a central authority
that securely distributes 100 key packets, each with a different set of 99 keys.
Either of these is possible, though tricky, for 100 users. Either becomes an administrative nightmare for larger
numbers. Moreover, keys must be changed regularly, so the problem of key distribution comes up again and
again. If you use the same key for many messages then an attacker has more text to work with in an attempt to
crack that key. Moreover, one successful crack will give him or her the text of all those messages.
In short, the hardest part of conventional cryptography is key management. Today the standard solution is to
build a hybrid system using public key techniques to manage keys.
TLS
Transport Layer Security, a newer name for SSL.
Transport mode
An IPsec application in which the IPsec gateway is the destination of the protected packets, a machine acts as its
own gateway. Contrast with tunnel mode.
Triple DES
see 3DES
Tunnel mode
An IPsec application in which an IPsec gateway provides protection for packets to and from other systems. Contrast with transport mode.
Two-key Triple DES
A variant of triple DES or 3DES in which only two keys are used. As in the three-key version, the order of operations is EDE or encrypt-decrypt-encrypt, but in the two-key variant the first and third keys are the same.
3DES with three keys has 3*56 = 168 bits of key but has only 112-bit strength against a meet-in-the-middle attack, so it is possible that the two key version is just as strong. Last I looked, this was an open question in the research literature.
RFC 2451 defines triple DES for IPsec as the three-key variant. The two-key variant should not be used and is
not implemented directly in Linux FreeS/WAN. It cannot be used in automatically keyed mode without major
fiddles in the source code.
Virtual Private Network
see VPN
VPN
Virtual Private Network, a network which can safely be used as if it were private, even though some of its communication uses insecure connections. All traffic on those connections is encrypted.
IPsec is not the only technique available for building VPNs, but it is the only method defined by RFCs and supported by many vendors. VPNs are by no means the only thing you can do with IPsec, but they may be the most
important application for many users.
Wassenaar Arrangement
An international agreement restricting export of munitions and other tools of war. Unfortunately, cryptographic
software is also restricted under the current version of the agreement. Discussion.
X.509
A standard from the ITU (International Telecommunication Union), for hierarchical directories with authentication services, used in many PKI implementations.
CBS/ISS/VPN via the Internet, p. 40
6.3 Reference
RFC 2401 "Security Architecture for the Internet Protocol. S. Kent, R.Atkinson. November 1998. "
RFC 2402 "IP Authentication Header. S. Kent, R. Atkinson. November 1998."
RFC 2403 "The Use of HMAC-MD5-96 within ESP and AH. C. Madson, R. Glenn.November 1998. "
RFC 2404 "The Use of HMAC-SHA-1-96 within ESP and AH. C. Madson, R. Glenn.November 1998. "
RFC 2405 "The ESP DES-CBC Cipher Algorithm With Explicit IV. C. Madson, N.Doraswamy. November 1998. "
RFC 2406 "IP Encapsulating Security Payload "
RFC 2407 "The Internet IP Security Domain of Interpretation for ISAKMP. D.Piper. November 1998. "
RFC 2408 "Internet Security Association and Key Management Protocol"
RFC 2409 "The Internet Key Exchange "
What is a VPN – P.Ferguson and G.Huston - http://www.potaroo.net/papers.html
A Technical Guide to IPSEC Virtual Private Networks – James S.Tiller - ISBN 0-8493-0876-3